Reaction of tryptophan with carbohydrates: mechanistic studies on the formation of carbohydrate-derived beta-carbolines.

Numerous carbohydrate-derived beta-carbolines have been identified for the first time in model reactions of tryptophan with glucose and ribose, as well as in food samples. Extending these structural studies, we performed detailed investigations to elucidate the corresponding intermediates and formation pathway of these alkaloids. Degradation experiments with purified tryptophan glycoconjugates established that only glyco-tetrahydro-beta-carboline-3-carboxylic acids, and not the N-glycosides nor the C-glycoconjugates represented the direct precursors of carbohydrate-derived beta-carbolines. In addition, the significance of the oxidative decarboxylation reaction as the initial step for formation of 1-substituted beta-carbolines was proven. Finally, the stereochemistry of the carbohydrate-derived side chain was studied by means of CD spectroscopy and HPLC-CD experiments. These detailed stereochemical analyses yielded experimental evidence for the racemization steps required for formation of the carbohydrate-derived harman alkaloids and confirmed the proposed reaction pathway.